The central nervous system (CNS) includes several tissues and organs, such as the brain, the brain stem, and the spinal cord. Each of these organs and tissues is made up of a variety of different types of cells and subcellular structures, e.g., neurons, glial cells, dendrites, axons, myelin, and various membranes. The CNS is isolated from the external world by several membranes that both cushion and protect these organs, tissues, cells, and structures. For example, the membranes that form the blood-brain barrier protect the brain from certain contents of the blood. The blood-cerebrospinal fluid barrier protects other portions of the CNS from many chemicals and microbes.
Access to the CNS for some substances is provided by specialized active transport systems or through passive diffusion through the protective membrane into the CNS. Present methods for delivering desired therapeutic agents to the CNS are typically invasive. For example, a pump implanted into the chest cavity (an intracerebroventricular pump) can effectively deliver a variety of useful compounds to the brain. However, implanting such a pump requires surgery, which can entail a variety of serious complications. Certain compounds (e.g., epidural painkillers) can be injected directly through the protective membrane into the CNS. Such injection is, however, impractical for most medications. Better methods for administering desired agents to the CNS, brain, spinal cord, and lymphatic channels are needed.